1. Field of the Invention
The present invention relates to a capacity leveling circuit which is used for a battery group configured by connecting plural cells in series, in order to level the capacities of the cells.
2. Description of the Related Art
For example, a traction battery for an electric vehicle is configured by a battery group in which a large number of cells are connected in series to provide a desired high voltage. In such a battery system, there is a problem that, when the cells are varied in capacity, voltage, temperature, and the like, the reliability of the battery system is lowered. Therefore, the states of each of the cells, such as the capacity, the voltage, the temperature, and the like are detected to monitor whether the cell is in a given condition or not.
In order to detect the voltage of each of such cells, usually, the configuration shown in FIG. 7 is used. In the figure, for the sake of simplicity, only four cells are illustrated. Voltage dividing circuits P1 to P4 each of which is configured by connecting resistors RA and RB in series are connected between the output terminals of cells C1 to C4 and a ground line serving as one of common output lines, respectively. According to this configuration, the voltages of the cells C1 to C4 can be measured from voltages V1 to V4 appearing at the common junctions between the resistors RA and RB of the voltage dividing circuits P1 to P4, respectively.
When the voltage dividing circuits P1 to P4 are connected to the cells C1 to C4 as described above, there arises a problem in that the capacities of the cells are varied and hence the reliability of the system is lowered. This problem is caused by discharging currents i1 to i4 respectively flowing through the voltage dividing circuits P1 to P4. Namely, the discharging currents have relationships that, as shown in FIG. 7, the discharging current i1 flows through the cell C1 only, the discharging current i2 flows through the cells C1 and C2, the discharging current i3 flows through the cells C1, C2, and C3, and so forth. Consequently, a larger current always flows through the cells C1, C2, . . . which are nearer to the ground line. As a result, the capacity of each of the cells is reduced in a larger degree as the cell is nearer to the ground line.
In the case where the battery is of the type in which overcharge does not produce a serious problem, the capacities of the cells C1 to C4 can be leveled by charging the cell C1 the capacity of which is lowered by the largest degree, so as to be fully charged, and the other cells C2, C3, and C4 to be overcharged. In a battery of the type in which overcharge should be avoided, such as a lithium-ion battery, however, the capacities of cells cannot be leveled by the above-mentioned technique.